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Combination therapy. An occasional reminder that using seat predictors on current polling is stupid

August 11th, 2019

You see it happen regularly, on here and on twitter. A new opinion poll comes out, showing dramatic news. Immediately, we decamp to Electoral Calculus and Flavible, to discover that such a poll, if replicated at a general election, would produce a hung Parliament with Plaid Cymru the largest party, able to form a coalition with the Greens and Lady Sylvia Hermon. The oracles have spoken. “Cor blimey”, we expostulate.

Why do we do it? At a time when the polls are in ferment, we all intuitively know that the polls aren’t an accurate reflection of what is going to happen at the next election, even if that next election might be only three months away (disclosure: I’m still firmly betting against a 2019 election, but that’s by the by). Numerous commentators, including me, have pointed out that seat predictors just aren’t equipped to deal with the types of poll movements that we have seen since the last election.  

So in another attempt to try to scotch this practice, let me illustrate the futility of this from a different direction.  As at the time of writing, the most recent poll is from YouGov. It showed the Conservatives on 31%, Labour on 22%, the Lib Dems on 21% and the Brexit party on 14%. Plugged into Electoral Calculus this gives: Con: 348, Lab: 191, Lib Dems 53 and Brexit: 1 (Clwyd South, if you’re curious). The poll before that, from ComRes, showed Labour on 30%, the Conservatives on 29%, the Lib Dems on 16% and the Brexit party on 15%.  Electoral Calculus turns that into Labour 290, Cons 265, Lib Dems 38 and Brexit 3 (Boston & Skegness, Montgomeryshire and Carmarthen West and Pembrokeshire South this time).

Hmm. Let’s look at that YouGov poll from a different angle. It shows the Conservatives and Labour on a combined vote share of 53%. Electoral Calculus show them winning 541 of the 632 British seats. In 2017 they shared 83%.  In 2017 they shared 579 of those 632 seats. The predictor is suggesting that their votes are going to be very efficiently distributed to withstand mislaying 30% of the electorate between them.

As it happens, we can test the likelihood of this against reality. In 2017, there were just 24 seats in Britain (excluding the Speaker’s seat, which is a special case) in which the Conservatives and Labour shared less than 53% of the votes.  Now bear in mind that if the Conservatives and Labour tally 53% of the votes between them, we can expect them to get less than that mark in roughly half of the seats. Of those 24 seats, between them they won precisely one.

Now if I tell you that all bar seven of those seats were in Scotland, you might triumphantly tell me that invalidates my observation, because the SNP are dominant there and it’s a gigantic special case (I don’t agree, because in any constituency in which they are recording low combined levels of votes, there are going to be other parties in the mix). But Labour and the Conservatives didn’t win any of the other seven either.

There are seats, such as Orkney & Shetland and Ceredigion, where neither Labour nor the Conservatives are in contention. There are two horse races, such as Eastbourne or Glasgow Central, where one of the two main parties is essentially absent. But only in Gordon did the Conservatives take the constituency.

The tipping point seems to be 60%. Above the combined Labour/Conservative 60% mark, only five constituencies were won by a different party (the SNP in all five cases). Below that level, another party winning was the norm rather than the exception. Between them, they took just 14 of the 61 constituencies in that category in 2017. Bear in mind that once they record two thirds of the vote between them (as they did in all bar 88 constituencies in Britain), one of them is bound to take the seat and you realise the idea of the major parties being super-efficient with their votes on low vote shares is hard to reconcile with such data as we have.

What all this suggests is that if the two major parties’ support declines as far as polls currently suggest, their seat count is going to be very dependent on the distribution of their opponents’ vote. There is no particular reason to assume that their new opponents’ vote is going to be especially inefficient: unlike the Alliance in the 1980s, both the Brexit party and the Lib Dems seem to have particular hot spots (in the case of the Brexit party, the more run-down bits of the country, and in the case of the Lib Dems, the more affluent bits of the country). The lumpier that support for each of them, the more likely they are to convert that into seats.

What does this mean in practice? If Labour and the Conservatives achieve a combined 60% of the vote at the next election, as currently seems an entirely reasonable starting point, they will poll above that level in roughly half the constituencies and below that level in the other half.  We can expect them to take substantially all those where their combined polling is above 60%, but they will be doing very well (and way beyond how they did in 2017) if they take 50% of the seats where their combined polling is below 60%.

If you do the maths, you find on such polling that a combined total of 475 seats between Labour and the Conservatives would be very good going for them.  That’s 80 less than Electoral Calculus predict on the ComRes poll (where they have a combined poll share of 59%).

To be clear, this is not to knock Electoral Calculus, who do a great job. I’m trying to show just how hard their job is. And I’m trying to persuade you that when the polls shift in the way that they have, past certainties break down. At times like this, the only wise course of action is to have some honest doubt.

Alastair Meeks